The reduction in size of memory cells and other circuits which are required for high density dynamic random access memories (DRAMs) is a continuing goal in semiconductor fabrication. The manufacturing of electronic circuits involves the connecting of isolated devices through specific conductive paths. When fabricating silicon and other semiconductive material substrates into integrated circuits, it is necessary to isolate devices built into the substrates from one another. Electrical isolation of devices as circuit density increases is a continuing challenge.
One method of isolating devices involves the formation of a semi-recessed or fully recessed oxide in the nonactive (or field) area of the substrate. These regions are typically termed as "field oxide" and are formed by LOCal Oxidation of exposed Silicon, commonly known as LOCOS. One approach of forming such oxide is to cover the active regions with a thin layer of silicon nitride that prevents oxidation from occurring therebeneath. The unmasked or exposed field regions of the substrate are then subjected to a wet oxidation, typically at temperatures of around 1000.degree. C. for two to four hours. This results in field oxide growth where there is no masking nitride.
However at the edges of the nitride, some oxidant diffuses laterally. This causes the oxide to grow under and lift the nitride edges. Because the shape of the oxide at the nitride edges is that of a slowly tapering oxide wedge that merges into another previously formed layer of oxide, it has commonly been referred to as a "bird's beak". The bird's beak is a lateral extension of the field oxide into the active areas where the devices are formed. Although the length of the bird's beak depends upon a number of parameters, the length is typically 0.15 micron to 0.5 micron per side.
This thinner area of oxide resulting from the bird's beak provides the disadvantage of both not providing effective isolation in these regions, as well as unnecessarily consuming precious real estate on the semiconductor wafer. Accordingly, prior art techniques have been developed in the formation of field oxide which limit the size and corresponding lateral encroachment of a bird's beak into active area. One such prior art process pertinent to this invention is described with reference to FIGS. 1-10.
Referring first to FIG. 1, a prior art semiconductor wafer fragment in process is indicated generally with reference numeral 10. Such is comprised of a bulk substrate 12, a thermally grown layer 14 of SiO.sub.2 and an overlying nitride layer 15. The function of layer 14, referred to as a pad or first sacrificial oxide layer, is to cushion the transition of stresses between silicon substrate 12 and nitride layer 15. Nitride layer 15 will function as the masking layer for ultimate formation of the field oxide regions.
Referring to FIG. 2, nitride layer 15 has been patterned and etched as shown to form nitride masking blocks 16 and 17. A channel-stop implant would typically be conducted prior to removing the mask, but might also be conducted later in the process. The etch to produce nitride blocks 16 and 17 is somewhat selective to the oxide of layer 14. Therefore, the etch does result in the removal of a portion of pad oxide layer 14 in an uneven manner due to the inherent non-uniformity of the etch process. Blocks 16 and 17 have opposing side edges.
Referring to FIG. 3, the wafer is subjected to a wet isotropic etch to remove remaining portions of exposed sacrificial oxide layer 14 from the substrate. This also produces undercut etching of layer 14 beneath nitride blocks 16 and 17, as shown.
Referring to FIG. 4, the wafer is subjected to oxidizing conditions to grow a second sacrificial oxide layer 18 having a thickness of from 60 Angstroms to 120 Angstroms. Layer 18 will function as a silicon etch stop, as will be apparent subsequently. Additionally, the thickness of layer 18 will affect the bird's beak size. The thicker layer 18 is, the larger will be the bird's beak size after field oxidation.
Referring to FIG. 5, a layer of polysilicon or amorphous silicon is deposited to a typical thickness of from 300 Angstroms to 1500 Angstroms. It is subsequently subjected to an anisotropic dry etch to form the illustrated silicon sidewall spacers 20 over the edges of masking blocks 16 and 17. Layer 18 serves as an etch stop to the underlying silicon during the spacer etch. Spacers 20 provide the desired effect during field oxidation of producing an oxide encroachment barrier to beneath masking blocks 16 and 17, such that the length of the bird's beak is substantially reduced.
The effect is best seen in FIG. 6. Wafer 10 is subjected to conventional wet oxidation techniques which cause silicon sidewall spacers 20 and the regions of substrate 12 not covered by blocks 16 and 17 to be oxidized to produce field oxide regions 22. The silicon of sidewall spacers 20 precludes significant oxygen encroachment to underneath adjacent nitride blocks 16 and 17 such that the illustrated birds' beaks 23 are much shorter in outward lateral expanse. Silicon sidewall spacers 20, being of a silicon material similar to substrate 12, are also oxidized during the process, essentially growing in volume to approximately twice their original size. This results in formation of what is commonly termed as "Mickey Mouse" ears 24. Unfortunately, these ears create subsequent processing problems apparent from FIGS. 7-10.
Subsequent to field oxide formation, nitride blocks 16 and 17 are stripped and wafer 10 then subjected to an oxide etch to remove second sacrificial oxide layer 18 (FIG. 7). This results in the "Mickey Mouse" ears acquiring a somewhat more sharp or pointed profile 24a.
During the previous growth of field oxide 22, an undesired phenomenon occurs that can cause defects in a subsequently provided gate oxide layer. Specifically, a thin layer of silicon nitride (not shown) can form on the silicon surface at the pad oxide/silicon interface as a result of a reaction of NH.sub.3 and silicon at that interface during field oxidation. This NH.sub.3 diffuses through the field oxide and the pad oxide, and reacts with the silicon substrate to form silicon-nitride spots or ribbons (not shown) which significantly adversely impact subsequent gate oxidation growth. The typical way of removing or attending to these silicon-nitride spots is by growth of a third sacrificial oxide layer 25 (FIG. 8). This layer must be removed, typically by a wet oxide strip. This unfortunately results in further etching of the field oxide regions 22 in such a manner that even more sharp upper topography points 24b are provided (FIG. 9).
These points undesirably result in increased topography, which can create photolithographic and etch difficulties in subsequent steps where polysilicon material for gate formation is provided. For example, FIG. 10 illustrates provision of a gate oxide layer 28, and subsequent provision of a polysilicon gate layer 27. Layer 27 has considerable higher topography over field oxide regions 22 than would be provided by more conventional field oxidation techniques. Accordingly, the advantage of achieving low lateral encroachment of birds' beaks of the prior art comes with a cost of adverse problems created by increasing
topography, and the problems associated therewith in semiconductor wafer processing.
It would be desirable to overcome such prior art drawbacks.